1. Field of the Invention
This invention relates to vehicle navigation systems and especially to the route guidance portion of such systems.
2. Description of the Related Art
Vehicle navigation systems are well known; see Yamada, U.S. Pat. No. 4,926,336 incorporated herein by reference disclosing a route searching system using a database describing road information including road intersections. See also Link et al. U.S. Pat. No. 5,270,937, incorporated herein by reference, disclosing a vehicle navigation/position system which searches road segments stored in a road map data memory for determining vehicle position in respect to road map coordinates.
The problem addressed by the present invention occurs e.g. when a vehicle navigation system is in an automatic route-searching mode. As shown in FIG. 1, the optimum route (the guided route NVRT) found by the system is continuously compared to the actual route being travelled by the vehicle. If there is a deviation from the guided route NVRT, the car is regarded to be off the guided route NVRT (i.e. on an "off-route" OFT). FIG. 1 shows a small portion of a road network which is stored in the database and which is also referred to as a set of map data. The database is conventionally stored as a series of road segments connecting nodes. The nodes are typically intersections of two road segments, but may be where a road segment terminates, for instance at a political or other boundary. In FIG. 1 the guided route NVRT is from the starting point node STP along the road segments which include nodes PT1 and RP1 to RP2 and ending in destination node DSP.
Of course it is to be understood that the route guidance of the system only provides instructions to the driver; the driver is by no means constrained to drive only along the guided route. For instance, the driver may be confused at a particular intersection by inadequate instructions from the system, or forced to detour by road conditions, and hence deviates from the guided route NVRT. In this case it is desirable for the system to guide him back to the guided (intended) route NVRT so that he may reach his destination DSP.
When an off-route condition is detected, and if the navigation system is in the conventional automatic route-searching mode, the system immediately searches for a node PT1 (entry point) which is an "unpassed" node on the guided route NVRT, to which the "slant distance" (a straight line in terms of latitude and longitude co-ordinates) from the present detected actual vehicle position Pc is the shortest distance (I1), searches for a route to navigate the vehicle to a return point RP1 on the original guided route NVRT from the present vehicle position Pc, and guides the vehicle according to the searched route to the return point RP1 which is on the original guided route NVRT.
Problematically, in conventional route-searching, the system searches the route from both the vehicle position Pc and the return point RP1 and finds a route from the current vehicle position Pc to the return point RP1, for example, the route Pc.fwdarw.CP1.fwdarw.RP1. In this case, since the return point RP1 is on the original guided route NVRT, the new guided route to the destination DSP (after recalculation of the guided route) is Pc.fwdarw.CP1.fwdarw.RP1.fwdarw.DSP. However, when the guided route is a winding route as shown, a route via a node close to the return point node PT1 to which the slant distance from the vehicle position Pc is the shortest, is not necessarily the shortest route to the destination DSP. Namely in FIG. 1, the route Pc.fwdarw.CP2 .fwdarw.RP2.fwdarw.DSP is shorter than the route Pc.fwdarw.CP1.fwdarw.RP1.fwdarw.DSP. However, the conventional system will not find this shorter route.
The same problem occurs in the conventional non-automatic route-searching mode of the system. The system determines a return point when the driver commands it to search for a new route for navigating the vehicle from the off-route vehicle position Pc to the return point RP1, and guides the vehicle to the return point RP1, and the system guides the vehicle according to the searched route to the guided route. This would occur when the driver for instance realizes he has departed from the original guided route NVRT, and commands the system to search for a new route to the destination.